**4.1 Physical treatments**

The removal of unstable white wine proteins could be performed by the use of ultrafiltration [5, 20, 87–90], flash pasteurisation [91–93], high hydrostatic pressure [10] and ultrasound [94].

Hsu et al. [20] ultrafiltered a white Gewürztraminer and Riesling wine with Romicon and Millipore systems, worked with membranes of nominal molecular weight cut-offs (MWCO) of 10–100 kDa. According to these authors, protein stability could be achieved with MWCO of 10 and 30 kDa; nevertheless, if the protein stability was not achieved, bentonite required was reduced from 80 to 95%. However, according to Miller et al. [95] and Flores et al. [87, 88, 96], ultrafiltration could also lead to the depletion of wine aroma compounds responsible for the floral, fruity and honey/caramel descriptors (**Table 1**), changing, in this manner, the wine aromatic profile [97, 98]. Additionally, wines treated by ultrafiltration also showed a significant reduction in yellow colour (420 nm) and total phenols [87, 88], as well as a decrease in the 'body' and 'mouthfeel' related to the removal of colloids [99]. Furthermore, the high operation and equipment cost associated with the aroma decrease, making this procedure unattractive to the wine industry for eliminating unstable proteins.

Wines heat treatments at medium temperature (45°C, several hours) and high temperature (90°C, 1 minute), with and without the application of proteolytic enzymes, lead to a decrease of the wine protein level and up to 70% of the bentonite needed for heat stability [91]. However, after sensory assessment of the wines submitted to the different treatments the panel members in some wines submitted to heat treatment without enzyme application and to heat treatment with enzyme application (Trenolin blank, 10 mL/L), observed slight effects on wine aroma descriptors [91].

The results obtained by Tabilo-Munizaga et al. [10] established that highpressure treatments changed the β-sheet and α-helical structures of wine proteins. During 60 days' storage period, the α-helix structure in high-pressure treatment samples was reduced. Structural modifications by high-pressure treatments (450 MPa for 3 and 5 minutes) increase wine proteins thermal stability and consequently delay the wine haze formation throughout wine storage.


#### **Table 1.**

*Mean scores\* of the significant aroma descriptor ratings for white Riesling and white Gewürztraminer wine (adapted from [97]).*

Recently, Celotti et al. [94] developed a research work focused on the application of ultrasound for white wine protein stabilisation. The results showed that higher amplitude (90%) and treatment time (10 minutes) induced an increase in white wine protein stability. This effect is related to the protein charge neutralisation and surface electrical charges, intending positive conformational modifications in the wine proteins. This technique could be considered as a way to prevent wine protein precipitation and to decrease the amount of bentonite fining agents used in wineries.
